(1) Field of Invention
This invention relates to an apparatus for heat processing of wafer-like objects, and more particularly to rapid thermal processing devices for the controlled manufacture of semiconductor type components.
(2) Prior Art
After the advent of the semiconductor industry some forty years ago, came the development of "tube" furnaces, which have been utilized to fabricate semiconductor devices from wafers of materials such as germanium, silicon and gallium arsenide. These tube furnaces were originally of horizontal orientation, consisting of an elongated heater positioned around a long cylindrically shaped tube, usually fabricated of quartz, with process gases entering one end of the tube and exiting at the other end. Tube furnaces have also been used for processing solar cells and non semiconductor materials for other applications. Tube furnaces are typically used with an atmospheric pressure ambient for standard processes and in a vacuum mode for when they are utilized in chemical vapor deposition processes.
Tube furnaces inherently require a long time to complete a process, because of the thermal inertia due to their large mass and their typical batch processing of wafers, but also because the furnace must be cooled down from its high operating temperatures to a lower temperature for loading and unloading of the wafers, to avoid thermal stresses on those wafers. In the early 1980's, the first rapid thermal type processors were developed as monitoring instruments, to serve as a fast way of processing "control" wafers by which the results of a tube furnace process called post-implant annealing (temperatures of 1000-1200 degrees C.) could be predicted.
This more rapid thermal processing (heating and cooling) of wafers disclosed many inherent advantages, which included the generic advantage of minimizing any shift of the semiconductor junction in the wafer. These early rapid thermal processors were driven towards speed as an end to itself, wherein high powered lamps were the preferred heating technique. Examples of some of these processors are described below. Most of the results with these techniques have been proven to be unsatisfactory, because of the lack of uniformity and consistency in the finished products, and the necessity for frequent adjustment and lamp replacement. These disadvantages have slowed the acceptance of rapid thermal processing as a replacement to tube furnace processing.
To hold a uniform temperature across a wafer which is essentially suspended in a free space--as it is being heated--and then cooled, using a high powered light flux, is extremely difficult, because of the many transients that occur as a consequence of large power changes that may take place in a few seconds. The lamps are expensive and do have to be replaced quite often because of their degradation through use. Also, in chemical vapor deposition, byproducts such as amorphous silicon, ammonium chloride and silicon dioxide build up upon the window between the lamps and the wafer being treated, thereby changing the heat flux upon the wafer.
An example of the use of lamps and their use is shown in U.S. Pat. No. 4,755,654 to Crowley et al. A quartz window is disposed between the lamp and the wafer being treated to modify the distribution of the light (heat) onto the wafer.
An earlier example of a tube type heating apparatus is shown in U.S. Pat. No. 4,041,278 to Boah. This patent shows a tubular quartz glass chamber with an infrared radiation source. This device utilizes infrared lamps to heat wafers optically for heating one side of a wafer.
A more recent device for treating wafers with chemical vapor deposition is shown in U.S. Pat. No. 4,796,562 to Brors et al. This apparatus utilizes cylindrical lamps to heat the wafer as uniformly as possible, in a flowing gas stream. Inherent in these lamp heated devices, is the problem of controlling the optical features which effects the uniformity of the product.
U.S. Pat. Nos. 4,081,313; 4,047,496 and 4,496,609 to McNeilly et al each show a reactor with a bank of high intensity lamps as a heat source.
Still yet another processor for wafer semiconductor processing is shown in U.S. Pat. No. 4,857,689 to Lee. This device comprises an elongated structure with the top enclosed within a heater and with a temperature gradient from top to bottom. A wafer is brought vertically toward the heated section. An optical sensor is arranged at the lower side of the wafer being treated, to regulate the amount of vertical travel of the wafer, up towards the top of the elongated structure until the wafer achieves the desired temperature. This device is an improvement over the thermal processors aforementioned, because of its temperature uniformity and repeatability, but its hot walls, which are an integral part of the device, become coated with deleterious byproducts of any chemical vapor deposition process. Furthermore, the large volume of the elongated structure is not readily adaptable to sequential processing. More importantly, the device does not adapt as well to various sizes and shapes of the objects to be processed as does the present invention.